The present invention concerns a polymer alloy, filaments made thereof, and fabrics made therefrom, which alloy is comprised of two compatible polymers having sufficient interfacial adhesion so as to remain bonded together as an extrudate, characterized in that one of the two component polymers has a higher melting point temperature than the second. Industrial fabrics in which at least a portion of the component filaments are formed from the polymer alloy exhibit reduced susceptibility to curl along their longitudinal edges.
Industrial textiles are well known and have a variety of uses, including carpeting, filtration and papermaking. Industrial textiles which are used in papermaking machines to drain and form the incipient paper web, known in the art as forming fabrics, must simultaneously possess a number of physical characteristics for them to be of value. At a minimum, they must be: resistant to abrasive wear, structurally stable, resistant to dimensional changes due to moisture absorption, resistant to stretch and edge curl under tension, as well as resistant to chemical degradation caused by the various materials present in both the stock and in cleansing solutions which are used to clean the fabrics at the prevailing temperatures of use.
Of the various polymers available for use in forming filaments intended for industrial textile applications, those most commonly used in papermaking fabrics are: polyesters, in particular polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), and their various copolymers; and polyamides, particularly polycaprolactam or nylon-6 (hereinafter referred to as polyamide-6), polyhexamethylene adipamide or nylon 6/6 (hereinafter referred to as polyamide-6/6), poly(hexamethylene sebacamide or nylon-6/10 (hereinafter referred to as polyamide-6/10), poly(11-aminoundecanoic acid) or nylon-11 (hereinafter referred to as polyamide-11) and poly(hexamethylene dodecanoamide) or nylon-6/12 (hereinafter referred to as polyamide-6/12); other polyamides are known and used.
Although filaments formed from both polyesters and polyamides are suitable for many industrial textile applications, the physical properties of both polymers can be improved, especially when used in the manufacture of industrial textiles intended for modern, high speed papermaking conditions. Polyester filaments generally provide adequate chemical and dimensional stability, and have good crimping and heatsetting characteristics which make them amenable to the weaving and finishing of industrial textiles; however, their resistance to abrasion could be improved so as to increase the service life of the fabrics into which they have been incorporated. Although polyamides have adequate properties for many applications, polyamide filaments have serious deficiencies for weaving and finishing as they exhibit poor crimpability and heatsetting behaviour, and generally do not possess adequate dimensional stability in the moisture range found in the paper making environment.
A further problem is that forming fabrics woven from either polyester, or alternating polyester and polyamide filaments, are subject to edge curl, a phenomenon in which the longitudinal edges of the fabric will either curl up and out of the plane of use, or will curl downwards and run in abrasive contact with the various stationary elements of the papermaking machine. This phenomenon is frequently observed in textiles following their weaving and removal of the fabric from the loom; it is particularly undesirable in forming fabrics which must be flat and generally macroscopically planar when in use so as to form the sheet uniformly, and resist wear along their marginal edges. Edge curl persists in the textile following heatsetting, and is well documented in the patent literature.
Heatsetting is a process used to stabilize a woven or nonwoven textile structure so as to set filament crimp and thereby prevent any deformation of the textile when in use. This is typically accomplished by applying heat to the fabric while it is under tension in at least one direction; the heat will soften the component filaments and lock them in position about one another during cooling. The temperature to which the fabric is heated during the heatsetting process will normally lie between the glass transition temperature and the melting point temperature of the component filaments. The applied tension and heat will cause the filaments to be permanently deformed and crimped about one another at their cross-over points. However, it is quite common for the edges of the textile to remain curled to some extent either up or down out of the plane of the fabric following this heatsetting process, necessitating further treatment (usually of the edges only) in order to render the textile usable.
Numerous attempts have been made to overcome the problems associated with both edge curl and dimensional stability of polyamide filaments in industrial textiles. These can be broadly broken down into mechanical means, chemical and heat treatment means. In the following discussion and hereinafter, the term xe2x80x9cfilamentxe2x80x9d is intended to be construed as synonymous with the terms xe2x80x9cyarnxe2x80x9d, xe2x80x9cfiberxe2x80x9d, xe2x80x9cmonofilamentxe2x80x9d and the like which are common in the textile arts and which are intended to denote a fundamental unit used in the construction of an industrial textile, generally, a fiber of indefinite length.
There are several known mechanical means to accomplish this objective. One method disclosed in U.S. Pat. No. 4,941,239 requires sanding off approximately xc2xc of the fabric mass from its outer, paper side surface edges.
Another method disclosed in U.S. Pat. No. 5,546,643 requires cutting slits into the paper side weft yarn knuckles along the lateral edges of the fabrics. This scoring of the weft yarns reduces the ratio between the cross machine direction shrinkage forces acting on the sides of the fabric to reduce the tendency to curl.
Another method known from U.S. Pat. No. 4,452,284 to reduce edge wear and curl is to weave the warp yarns along the longitudinal edges of the fabric at a lower tension than those in the central portion, or to utilize yarns at the lateral edges which are capable or greater elongation than those used in the central portion, such as polyamide edge yarns and polyester central yarns.
Another method proposed in WO 99/00546 to control edge curl is to score and notch the weft yarns along the longitudinal edges of the fabric by means of an ablation laser.
Yet another method disclosed in U.S. Pat. No. 4,453,573 is to utilize a modified, conventional unbalanced weave wherein every second warp pattern is reversed so as to improve fabric drainage and sheet support, as well as eliminate edge curl.
All of these mechanical methods that sand or score the fabric have an adverse impact on fabric life. As well, the methods which include specialized weaving requirements require additional manufacturing time and/or specialized equipment.
It has also been proposed to reduce fabric edge curl by chemical means. U.S. Pat. No. 5,324,392 teaches that monofilament made from a unique polyamide-6 which is manufactured in the specified manner provides good crimp and shrinkage characteristics. These yarns are used as at least the weft yarns in weaving single or multi-layer forming fabrics which are allegedly stable and resist edge curl. Specialized polyester monofilaments have also been proposed in U.S. Pat. No. 5,116,478 for the same intended end use which have similar crimp and shrinkage characteristics, as well as good wear resistance properties.
Another proposal to control edge curl is disclosed in U.S. Pat. No. 4,281,688 wherein the use of a weave pattern which introduces weft yarn floats and/or knuckles of differing sizes is taught.
GB 2,328,452 discloses controlled cooling of industrial textiles by means of a blower located immediately downstream of the heatsetting chamber, or following the return roll, so as to provide a uniform flow of cooling air across the fabric surface to minimize fabric distortion and edge curl following heatsetting.
None of these aforementioned teachings has met with complete success in eliminating edge curl in industrial textiles. One common means of reducing fabric edge curl is to increase the temperature at which the textile is heatset, at least at its lateral edges, so that it is close to the melting temperature of the component yarns. This practice is somewhat effective, however, other desirable physical properties of the textile, such as its finish, surface characteristics, permeability to air and fluids, and resistance to hydrolytic degradation, may be significantly diminished. It would therefore be desirable to provide a woven textile and, in particular, a paper making fabric, that is not subject to edge curl, can be produced on conventional looms, and which provides adequate textile properties for the intended end use application, in particular with respect to the crimpability of the component yarns, heatsetting behaviour of the textile and its resistance to abrasive wear.
In a first broad embodiment, the present invention provides a polymer alloy formed from first and second polymers which are mutually compatible and which exhibit sufficient interfacial adhesion so as to remain bonded together following mixing, melting and extrusion. The polymer alloy is comprised of a first polymer having a first, higher temperature melting point and a second polymer having a second, lower temperature melting point. The first and second polymers are mixed so that upon blending and melting, two distinct melting points are observed in the polymer alloy extrudate, and the extrudate remains stable at a temperature which is lower than the first, higher temperature melting point but which is higher than the second lower temperature melting point so as to allow permanent plastic deformation of the extrudate. Preferably, the first higher temperature melting point is at least 5xc2x0 C. greater than the second lower temperature melting point. The first and second melting point temperatures are preferably determined by means of Differential Scanning Calorimetry (DSC); other methods may be suitable. When this preferred method is used, the melting points of the polymers in the alloy are defined by the peaks of the heat flow/temperature curve provided by the DSC apparatus.
In a second broad embodiment, the present invention provides a synthetic filament formed from a polymer alloy comprised of first and second polymers which are mutually compatible and which exhibit sufficient interfacial adhesion so as to remain bonded together following mixing, melting and extrusion. The first polymer has a first, higher temperature melting point and the second polymer has a second, lower temperature melting point. The first and second polymers are mixed so that following blending, melting and extrusion of the polymer alloy in filamentary form, two distinct melting points are observed in the resulting extrudate. The extrudate will remain stable when exposed to a temperature which is lower than the first, higher temperature melting point and which is greater than the second, lower temperature melting point. Preferably, the first higher temperature melting point is at least 5xc2x0 C. greater than the second lower temperature melting point as determined by DSC.
In a third broad embodiment, the present invention provides an industrial textile formed from a machine direction (MD) yarn system interwoven with a cross-machine direction (CD) yarn system, wherein at least one of the MD and CD yarn systems includes a filament formed from a polymer alloy of first and second polymers which are mutually compatible and which exhibit sufficient interfacial adhesion so as to remain bonded together following mixing, melting and extrusion. The first polymer has a first, higher temperature melting point and the second polymer has a second, lower temperature melting point. The first and second polymers are mixed so that following blending, melting and extrusion of the polymer alloy in filamentary form, two distinct melting points are observed by DSC in the resulting extrudate. The extrudate will remain cohesive when exposed to a temperature which is lower than the first, higher temperature melting point and which is greater than the second, lower temperature melting point. Preferably, the first higher temperature melting point is at least 5xc2x0 C. greater than the second lower temperature melting point when determined by DSC.
Industrial textiles into which these filaments are incorporated as at least a portion of either, or both, the interwoven MD or CD yarn systems are heatset at a temperature that is at least equal to, and is preferably greater than, the second, lower melting point temperature, but which is lower than the first higher temperature melting point. The resulting fabrics exhibit reduced propensity for edge curling when compared to comparable fabrics of the prior art, and are dimensionally stable and resistant to abrasive wear when in use.
Preferably, the polymer alloy of the present invention is comprised of a first polymer whose higher temperature melting point is greater than 200xc2x0 C. and a second polymer whose lower temperature melting point is less than 200xc2x0 C., both temperatures being determined by DSC. Alternatively, the first polymer has a higher temperature melting point which is greater than 190xc2x0 C. and a second polymer has a lower temperature melting point is less than 190xc2x0 C., both temperatures being determined by DSC. As a further alternative, the first polymer has a higher temperature melting point which is greater than 180xc2x0 C. and a second polymer has a lower temperature melting point is less than 180xc2x0 C., both temperatures being determined by DSC.
As used herein, the phrase xe2x80x9cmelting point temperaturexe2x80x9d refers to the actual temperature at which, in a semi-crystalline polymer, the last traces of crystallinity disappear under equilibrium conditions and the polymer melts and flows. In the preferred embodiments described herein, all of the polyester and polyamide polymers are of the semi-crystalline type. All melting point temperatures provided are determined by means of Differential Scanning Calorimetry (DSC). The melting point temperatures of the polymers are represented by a peak in the heat flow/temperature graph.
Preferably, the polymer alloy of the present invention is comprised of from 50% to 99% by weight of the first higher temperature melting point polymer, and from 1% to 50% by weight of the second lower temperature melting point polymer, with the percentages by weight being based on the total weight of the polymer system.
In one particular embodiment which is presently preferred, the first higher temperature melting point polymer is polyamide-6/10 and the second lower temperature melting point polymer is polyamide-11.
In a second particular embodiment which is also presently preferred, the first higher temperature melting point polymer is polyamide-6 and the second lower temperature melting point polymer is polyamide-11.
In a third particular embodiment which is also presently preferred, the first higher temperature melting point polymer is polyamide-6/12 and the second lower temperature melting point polymer is polyamide-11.
Preferably, the melting point temperatures of the first and second polymers are determined in the polymer alloy by means of Differential Scanning Calorimetry (DSC).
It will be understood by those of skill in the art that other polymers which are compatible, exhibit sufficient interfacial adhesion so as to remain bonded together, and which have differing melt points, may also be used to form the polymer alloy of the present invention.